1922] Kellogg: Calif ornian Forms of Microtus Montamus 261 



width usually not more than twice inferior width. The plate forming 

 the outer wall, and continuous with the lacrimal internally, present, 

 though incomplete superiorly. 



Occiput not depressed, the median depth averaging more than 

 59 per cent of greatest width across lambdoidal ridge of brain case. 

 Contour of foramen magnum variable, occasionally somewhat flattened 

 superiorly, with vertical diameter usually less than, sometimes e(iual 

 to, transverse, Paroecipital processes heavy, directed more downward 

 than backward, with their extremities applied to bullae. Posterior 

 margins of paroecipital processes continued upward as well defined 

 ridges on supraoccipital though seldom reaching lambdoidal ridge 

 superiorly. Basioccipital elevated and with well defined median ridge. 

 Width of basioccipital at suture between it and basisphenoid usually 

 less than one-third its median length. Auditory bullae well rounded, 

 and rather large, nearly equaling those of MicroUus ealifornicus 

 aestuarinus in old individuals. Posterior border of bulla rather 

 oblique, very slightly indented by fenestra rotunda. 



Palate rather narrow, with palatine sulci shallow and indistinct. 

 Median ridge variable, but lateral bridges well developed. Lateral 

 pits of palate large and fenestrated. Pterygoids and hamular pro- 

 cesses gradually diverging posteriorly. Interpterygoid fossa relatively 

 narrow as comJDared with ealifornicus, with posterior margin of palate 

 variable, normally rather squarely truncate, occasionally rounded. 

 Posterior palatine foramina small. Incisive foramina long, narrow 

 and constricted posteriorly, equalling about 64 per cent of distance 

 between base of incisors and anterior border of alveolus of first pre- 

 molar. Zygomatic arches divergent anteriorly, relatively heavy, with 

 median portion nearly parallel to main axis of skull and slightly 

 indented at junction with premaxillae. 



Mandible similar to that of ealifornicus. Ridge for masseteres 

 laterales prominent and well developed; Coronoid process large, its 

 base broad, its extremity rising above level of condyle and curved 

 strongly backward. Angular process rather wide, with its main axis 

 curving outward and upward. Articular process over base of incisor 

 marked by well developed protuberance. Base of mandibular foramen 

 on a level with or slightly above cutting surface of last lower molar. 

 Mental foramina on outer face below superior surface of diastema. 



Teeth. — Premolar and molars relatively light, their enamel pattern 

 well defined, all the elements distinct, with deep salient and reentrant 

 angles. Fourth upper premolar is not noticeably longer than first 

 molar and is of approximately the same length as second molar. 

 First upper molar with four closed sections. Fourth lower premolar 

 normally with six closed triangles (fig. F). 



Fourth upper premolar normally with an anterior transverse loop 

 and four alternating closed sections; two deep reentrant angles are 

 normally present on each side ; these are sharply pointed, their points 

 extending across long axis of tooth, those of outer side deeper than 

 those of inner side ; anterior loop with enamel enclosing protoconule 

 normally flattened superiorly and shorter than same element of 

 parastyle, its long axis at nearly right angles to that of tooth row. 

 Triangles forming protocone and hypocone normally with sharper 

 apex than triangles forming paracone and metacone. Loop forming 

 metaconule with sharp internal angle. 



